The principle of selective photothermolysis underlies many laser therapies and is used to treat such diverse dermatological problems as leg veins, portwine stain birthmarks, other ectatic vascular lesions, and pigmented lesions including tattoos. The dermal and epidermal layers containing the targeted structures are irradiated with light, usually from lasers or flashlamps. The wavelength or color of this light is chosen so that its energy will be preferentially or selectively absorbed in the structures. This leads to the localized heating with the intent of raising the temperature to a point at which constituent proteins will denature or pigment particles will disperse.
The pulse duration of the irradiating light is also important for selectivity. If the pulse duration is too long, heat absorbed by the structures will diffuse out into the surrounding tissues and will not be selectively heated to the degree necessary. If the pulse durations are too short, however, the light absorbing chemical species such as blood hemoglobin or tattoo dye particle will be heated too quickly causing vaporization. Theory dictates that the proper pulse width should match the thermal diffusion time of the targeted structures. For smaller vessels contained in portwine stain birthmarks, for example, these thermal diffusion times can be on the order of hundreds of microseconds (.mu.sec) to several milliseconds (msec). Larger leg veins have thermal diffusion times in the 5 to 100 msec range. Pigmented lesion particles can have diffusion times as short as nanoseconds (nsec).
Various types of lasers have been tested for selective photothermolysis in dermatological specimens. Q-switched alexandrite lasers have been successfully used to treat naturally occurring dermatological pigmentations and also tattoos. Long-pulsed ruby lasers have been proposed for the removal of hair. Nd:YAG lasers (operating at 1060 nm), carbon dioxide (operating at 10.6 micrometers), and argon (operating in the 488-514 nm range) have been suggested for the treatment of ectatic vessels. The most successful vascular treatments have been achieved using dye lasers, and specifically flashlamp-excited pulse dye lasers. These lasers operate in the 577-585 nm range where there are absorption band peaks for hemoglobin and also operate well in the pulsed mode that provides for good selectivity. With the proper selection of color and pulse duration, success rates of higher than 50% are common when treating smaller vessels. Unfortunately, dye lasers are limited in pulse durations to less than 1.5 milliseconds. Thus, they tend to be inappropriate for the treatment of larger structures that would require pulse durations of hundreds of milliseconds, at least according to the principle. Attempts are being made to solve this problem. Frequency doubling Nd:YAG has been proposed as a technique to generate long pulses at 532 nm.